The Panic Blog provides an excellent explanation of code-signing for dummies, and why the new Gatekeeper functionality in OS X is important.

You’ve probably heard at least in passing about encryption keys and that they have different sizes or lengths (such as 40-bit, 128-bit, or 256-bit). Just like in my number guessing example above, longer keys are harder to guess. Each additional bit that is added to a key makes it exponentially harder to guess or figure out by brute-force attempting to decrypt the data with every possible numerical key. (Is 1 the key? No. Is 2 the key? No. Is 3 the key? Is 3,426,989,662 the key? No.)